Coating workpiece method using beam of plasma

ABSTRACT

Method for coating workpieces generates a beam of a plasma in an evacuated container. A region of highest plasma density is at the beam axis and workpieces having surfaces to be coated, are radially offset from, and extend along the axis with the surfaces facing the axis and being in the container. Fresh reactive gas is inlet into the container and consumed gas is removed from the container. Coating material is deposited upon the surfaces with a deposition rate of at least 400 nm/min and at a maximum temperature of the surfaces being 550° C.

The present invention fundamentally has the goal of depositing materialswith reactive plasma-enhancement i.e. by a PE-CVD method, on adeposition surface, on the one hand with maximally high deposition rateonto the deposition surface, on the other hand, at minimally lowtemperature of this surface.

We define the deposition rate as the material thickness applied onto asurface per unit time, if said surface is disposed within the vacuumcontainer at a defined site yet to be explained, since, in particular inthe present context, the quantity of material deposited onto a surfaceunit per unit time is a function of the location at which the surface isdisposed.

It is known from “Plasma-Enhanced Chemical Vapour Deposition ofEpitaxial Silicon from Silan”, S. R. Shanfield et al., 1046 B, ExtendedAbstracts, Vol. 83-1 (1983), May, Pennington, N.J., USA; XP-002056339,to deposit epitaxial silicon layers by means of PE-CVD, with silane asthe reactive gas. Therein a substrate temperature between 700° and 900°results. Coating rates (FIG. 3) of maximally 40 nm/min are achieved.

From “Low Temperature Deposition of Microcrystalline Silicon in aMultipolar Plasma”, T. D. Maintei et al., 1046 B Extended Abstracts,(1985), October, No. 2, Pennington, N.J., USA; XP-002056340, is furtherknown to deposit microcrystalline silicon layers with a PE-CVD method atcoating rates up to approximately 40 nm/min at a surface temperature of100° C. and up to approximately 25 nm/min at a surface temperature of250° C.

From DE-OS 36 14 384 by the same applicant as the present application itis known to attain by means of a PE-CVD method using a low-voltagehigh-current arc discharge coating rates of 200 nm/min in the coatingwith nickel in Ni(CO)₄ gas as the highest deposition rate specifiedthere. Si is therein only deposited at a coating rate of approximately17 nm/min. With the aid of the low-voltage discharge a homogeneous denseplasma is generated in the vacuum container.

According to “Plasma-Assisted CVD of Diamond Films by Hollow Cathode ArcDischarge”, J. Stiegler et al., Diamond and Related Materials, 2 (1993),413-416, it is known to deposit diamond layers with a PE-CVD method at adeposition rate of up to approximately 35 nm/min at surface temperaturesof at least 700° C.

Furthermore known from “Low Temperature Plasma-Enhanced Epitaxy ofGaAs”, K. P. Pande, 1046 Journal of the Electrochemical Society, 131(1984), June, No. 6, Manchester, N.H., USA, is to deposit GaAs epitaxiallayers at low temperatures below 400° C., however, at deposition ratesof 80 nm/min, however only starting at temperatures about 500° C.

U.S. Pat. No. 5,554,222 discloses depositing diamond-like layers atrelatively cold deposition surface temperatures of 250° C. when cooled,and 400° C. when not cooled. Deposition rates of 20 nm/sec werereported.

From PCT/CH98/00221 by the applicant of the present application it isfurther known to attain by means PE-CVD deposition rates ofapproximately between 100 and 200 nm/min, depending on the depositionmaterials, at surface temperatures between 300° and 800° C.

It is the task of the present invention to propose a method for thereactive plasma-enhanced deposition of material onto a depositionsurface, by means of which at low temperatures of the depositionsurface, without it being cooled, significantly higher deposition ratesare attained compared to such prior known PE-CVD methods.

This is achieved through the use of a method for the reactiveplasma-enhanced treatment of workpieces, in which a plasma beam isgenerated in an evacuated container and workpieces are disposed radiallyoffset with respect to the region of highest plasma density along thebeam axis, with fresh reactive gas being allowed to flow into thecontainer and consumed gas being suctioned from the container andsurfaces to be treated identically are disposed equidistantly withrespect to the beam axis for the deposition of material on a depositionsurface with a material generation rate of at least 400 nm/min and at atemperature of maximally 550° C. However, significantly lowertemperatures are therein possible.

From EP 0 724 026 by the same applicant as of the present invention, amethod is known for the reactive treatment of workpieces in which aplasma beam is generated in an evacuated container, and radially offsetwith respect to the region of highest plasma density along the beamaxis, are disposed workpieces, wherein fresh reactive gas being allowedto flow into the container and consumed gas is suctioned from thecontainer, and in which, further, workpiece surfaces to be treatedidentically are disposed distributively about the plasma beam along alongitudinally extended surface of revolution, and specifically suchthat the plasma density on the surfaces is at most 20% of the maximumbeam plasma density, viewed in each instance in planes perpendicular tothe beam axis, which is suitable for depositing difficult to producemetastable layers, in particular of diamond, CBN, α-Al₂O₃ or C₃N₄layers. In this document it was found that the diffusion region ofhigh-current arc discharges, i.e. the region of a plasma density of ≦20%of the beam center plasma density, is extraordinarily well suited forthe deposition of extremely hard layers, in particular for thedeposition of layers out of metastable phases, such as the above,difficult of generation under normal conditions.

It was found according to the present invention that this advance is notonly suitable for the deposition of layers difficult to produce, but,surprisingly for the deposition fundamentally at very high depositionrates and, as stated, while maintaining low temperatures.

The use according to the invention is especially suitable for thedeposition of microcrystalline silicon, therein especially highlysuitable of μc-Si:H.

In particular in this use it was found that with the content of hydrogenin the process atmosphere the temperatures of the treated workpieces canbe set in a wide range, thus between temperatures above 400° C. down totemperatures above 250° C. The lower the hydrogen fraction, the lower issaid temperature. Since this H₂ content in the formation of μc-Si:H isnot particularly critical, this parameter is highly suitable as atemperature setting variable in particular when depositing this materialto be used.

It should be emphasized that based on the prior known methods for veryhard layers and in particular for said metastable phases which aredifficult to produce, it is by no means evident that this method issuitable for the high-rate deposition, on the contrary.

The plasma beam in highly preferred manner and as described in EP 0 724026, is developed as a low-voltage arc discharge, preferably as ahigh-current arc discharge.

In the use according to the invention, the deposition is used as acoating deposition or as deposition of the material in powder or clusterform, i.e. in the last cited case, to obtain said material powder orcluster. In order to carry out, furthermore, said deposition at amaximum degree of efficiency, i.e. utilized deposited quantities ofmaterial for each reactive gas quantity introduced, it is furtherproposed that the deposition surface is disposed along surfaces ofrevolution about the beam axis.

To attain maximally high efficiency with respect to the depositedquantities of material and introduced reactive gas, it is furtherproposed to dispose the deposition surface, be that developed by acollector surface for the deposited powder or cluster, be that formed byworkpiece surfaces to be coated, annularly about the axis of the plasmabeam.

In particular, if, as in the application of the use according to theinvention for surface coating, the deposition thickness homogeneity isan essential criterium, it is further proposed to rotate the depositionsurface about the beam axis and/or about an axis of rotation offset fromthe beam axis, preferably parallel hereto, during the deposition.

Homogenization of the deposition distribution is also attained through areactive gas flow in the container generated substantially parallel tothe beam axis.

In a further, highly preferred embodiment of the use according to theinvention, the plasma density distribution is controlled by means of amagnetic field generated substantially parallel to the beam axis. Ifsuch a field is applied, then preferably of maximally 250 Gauss,preferably of 100 Gauss, in particular preferred of 60 Gauss.

Depending on the application purpose, the deposition surface can beplaced at floating potential or at a preferably settable electricpotential, therein to a DC, an AC or an AC+DC potential.

The plasma beam is further in a preferred embodiment generated by meansof a low-voltage arc discharge with hot cathode or with cold cathode,preferably as a high-current arc discharge. Especially preferred andessential for the development in particular of thelow-voltage/high-current arc discharge the total pressure in thecontainer is maintaining at minimally 1 mbar.

The use according to the invention in the highly preferred embodiment isaiming for the deposition of microcrystalline silicon, in particular ofμc-Si:H, wherein preferably silane is employed as the reactive gas. Itis therein in particular also essential that according to the inventionmicrocrystalline silicon can be deposited in nm up to μm powder orcluster form. With said high deposition rate, furthermore, as layer orpowder, further silicon compounds can be deposited, such as SiC, SiN,but additionally also metal compound layers, such as in particular hardsubstance layer materials, such as for example TiN, TiAlN, SiAlON layersor layers with low coefficients of friction, such as CrC-, FeC-, WCClayers etc. In spite of the high deposition rates, in the deposition ascoating a high coating quality suitable for epitaxial layer formation isobtained.

Furthermore, with the use according to the invention industriallywidespread silicon or glass substrates are preferably coated.

In the following, the invention will be explained by example inconjunction with figures and examples, and, with respect to the priorknown advance applied according to the invention, reference is made tothe EP-A 0 724 026 forming an integrated component of the presentspecification. In the Figures depict:

FIG. 1 schematically, a high-current arc and the disposition accordingto the invention with respect to it of deposition surfaces,

FIG. 2 schematically an installation used according to the invention forthe preferred deposition according to the invention of microcrystallinesilicon.

In conjunction with FIG. 1, first the fundamental advance according tothe invention will be explained. The plasma beam preferably developed ashigh-current arc 1 diverges rapidly within a few cm after anozzle-opening of a cathode chamber 3 to a certain expansion in order toretain a largely constant development up to shortly before the anode, upto a few cm in front of it. The expansion of the arc before the anodedepends on the geometric form of the anode. Except for a small regionafter the diaphragm opening 2 and over the anode 4, a largelyhomogeneous long region 1 of the high-current arc consequently results.Along the axis A in the diametrical cutting plane E, such as plotted byexample, bell-shaped curve distributions of the plasma density result.The plasma density distribution has in each plane E a maximum Max.

For the use according to the invention the arc length is preferablybetween 50 and 120 cm, in particular preferred approximately 90 cm.

For the use according to the invention, further, the total pressure inthe container is selected to be greater than 1 mbar. The surfaces to beprovided according to the invention for the deposition of materials, arepreferably disposed in a radius r from the beam axis A, at whichmaximally a plasma density of 20% obtains.

The width of the plasma beam, and thus also the distribution of theplasma density, is set according to the particular requirements via thesetting of the arc current and/or in particular via an axial magneticfield H, as shown in FIG. 1.

The field strength is therein preferably set to maximally 250 Gauss, butin particular to maximally 100 Gauss, but especially highly preferredbetween 0 Gauss (without field) and 60 Gauss.

Due to the disposition of the deposition surfaces in the region of lowplasma density, the advantage is also obtained that this varies onlyslightly along straight lines parallel to the beam axis and in thehomogeneous region 1.

For this purpose the distance r of the workpiece surfaces to be treatedfrom the beam axis A is preferably selected to be

 6 cm≦r≦20 cm,

in particular to be

9 cm≦r≦13 cm,

As depicted further in FIG. 1, relatively large deposition surfaces tobe used, and in particular such surfaces on which a coating is to begenerated, are moved preferably oscillating or revolving with respect toaxes parallel to the beam axis, as shown with ω_(M), and/or optionallyabout the beam axis A, as shown with ω_(A). Furthermore, the reactivegas flow, as shown with G, is preferably generated substantiallyparallel to the beam axis A.

In FIG. 2 an installation according to the invention is shownschematically. The plasma beam 1 is preferably generated as ahigh-current arc discharge in a vacuum container 10, preferably in theform of a hot cathode low-voltage high-current arc discharge, howeverwherein also a cold cathode arc discharge can be employed.

Onto the vacuum container 10 is flanged a cathode chamber 12 with anelectron-emitting hot cathode 14, which is connected into a heatingcurrent circuit for the heating current I_(H) with preferably settableheating current generator 16.

Opposing a beam exit nozzle 18 provided on the cathode chamber 12, isdisposed the anode 20. Between a hot cathode 14 or the heating currentcircuit and anode the preferably settable discharge generator 22 isconnected. With respect to details of this known configuration,reference can be made to CH 664 768 in addition to EP-0724 026.

A workpiece support configuration 24 defining a cylindrical surface isprovided for deposition surfaces to be coated with the use according tothe invention or as a collecting surface for the deposition of materialpowder or clusters. The radius r, as was evident based on the aboveexplanations, depends on the beam power.

Along the cylindrical workpiece support configuration, while observingsaid plasma density conditions, workpieces to be coated, such as, in apreferred embodiment, glass or silicon substrates, are provided. Theyare provided in particular for the high-rate deposition ofmicrocrystalline silicon. But other workpieces, such as tools, forexample, drills, indexable inserts, milling cutters etc. can also bedisposed thereon in order to coat them with hard substance layers suchas layers comprised of TiN, TiAlN, Si—Al—ON, SiC, SiN, etc. or withlayers with low coefficients of friction, such as with layers comprisedof CrC, FeC, WCC, thus for example metal-carbon layers. In order toutilize the reactive gases as efficiently as feasible, optionallyprovided substrate supports are built to be as transparent as possiblesuch that the main surface is formed by the workpieces or substratesthemselves and not by their mounting support.

As can furthermore be seen in FIG. 2, reactive gas R, preferably silane,for the preferred embodiment, namely the deposition of microcrystallinesilicon is introduced at 29 into the container 10; at the anode side thepumping configuration 26 is provided. Thereby a gas flow directedsubstantially parallel to axis A through the container and along thedeposition surface 24 disposed on a cylindrical surface is set up. Via asupport configuration workpieces or generally deposition surfaces areoperated at a floating potential or connected to a reference potential,for example ground potential, or connected to a DC bias potential, an ACor a mixed AC+DC potential, such as for example to a pulsed DCpotential. This and further options of potential connections for thedeposition surfaces are shown in FIG. 2 schematically with thechange-over unit 28. The installation according to FIG. 2 was coated forthe examples 1 to 6 according to the following Table.

Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 ProcessParameters Arc current [A] 100 160 400 120 120 120 Magnetic field[Gauss] 50 80 60 0 0 30 Total pressure [mbar] 1.5 1.5 2.0 1.5 1.5 1.5Gas flows [sccm] Ar 1800 1800 1600 2700 2700 2700 H₂ 20 50 40 50 40 0SiH₄ 60 100 100 75 80 100 Distance from arc axis [cm] 16 10 16 7 10 9Substrate temperature [° C.] 250 350 450 450 400 420 Coating time [min]40 80 20 16 25 24 Growth rate [nm/min] 450 600 600 630 400 500 Arclength [cm] 90 90 90 50 50 50 Layer properties Layer thickness [μm] 1848 12 10 10 10 Crystal structure μc-Si:H μc-Si:H μc-Si:H μc-Si:H μc-Si:Hμc-Si:H

In all examples μc-Si:H structures were deposited, directed onto theemployed glass or silicon substrates. The layers had to some extentepitactic qualities. It should be noted that in none of the layers anamorphous intermediate layer was generated directly on the substrate.This is unusual since in conventional coating methods normally anamorphous intermediate silicon layer is initially always formed, whichsubsequently prevents epitaxial layers from growing on. This indicatesthat the method employed according to the invention is also suitable forthe deposition of epitaxial layers. Noticeable is further the extremelyhigh growth rate which is attained even at low deposition temperatures.

In general, for the use according to the invention the followingoperating parameters can be specified:

Arc current: 80-170 A Total pressure: 1 mbar ≦ P_(tot) ≦ 3 mbar Axialmagnetic field H: 0-250 Gauss, preferably 0-100 Gauss, especiallyadvisable 0-60 Gauss.

Distance r from the beam axis to the workpiece surface:

6 cm ≦ r ≦ 20 cm, preferred is 9 cm ≦ r ≦ 13 cm.

For the preferred application, namely the deposition of microcrystallinesilicon, further, at a container size of approximately 80 l and an arclength, dimensioned as specified, of 90 cm according to the aboveexamples, the following gas flows were employed:

Argon:  1800 sccm H₂: 0-100 sccm Silane, SiH₄: 5-100 sccm

Substrate temperatures were attained which are not higher than 450° C.,customarily between 250 and 500° C.

Depending on the set arc current, to maintain the conditions of plasmadensity, preferably of maximally 20% of the maximal density at the beamaxis, radial distances are obtained from the beam axis between 6 and 20cm, preferably, as stated, of 9 to 13 cm.

Depending on the setting of the operation parameters, in particularduring the deposition of microcrystalline silicon, said layers areobtained as coatings or as deposition in the form of powder or clustersof nm to μm size.

As a function of the hydrogen content of the treatment atmosphere,further, at a total pressure of 1.5 mbar, without a magnetic field beingapplied, further with r=10 cm, an arc current of 120 A as well as aconstant argon flow of 1800 sccm, the following relationship wasmeasured:

H₂ flow (sccm) Temperature at deposition surface (° C.) 10 220 30 270 50310 70 350 100 390 150 450

Based thereon, it is evident that the hydrogen content in the treatmentatmosphere is extraordinarily well suited to set the depositiontemperature at the workpiece surfaces.

What is claimed is:
 1. A method for coating workpieces comprising thesteps of: generating a beam of a plasma in an evacuated containerhaving, along an axis of said beam, a region of highest plasma density;disposing workpieces having surfaces to be coated, radially offset from,and extending along said axis with said surfaces facing said axis;inletting fresh reactive gas into said container and removing consumedgas from said container; and depositing coating material upon saidsurfaces with a deposition rate of at least 400 nm/min and at a maximumtemperature of said surfaces of 550° C.
 2. The method as claimed inclaim 1, including deposition the coating material as one of a powderand a cluster.
 3. The method as claimed in claim 1, wherein said plasmabeam is a low-voltage arc discharge.
 4. The method as claimed in claims3, wherein said plasma beam is a low-voltage and a high-current arcdischarge.
 5. The method as claimed in claim 1, including rotating saidworkpiece surfaces around said beam axis.
 6. The method as claimed inclaim 1, including oscillatingly pivoting said workpiece surfaces arounda further axis that is offset with respect to said beam axis.
 7. Themethod as claimed in claim 6, wherein said further axis is parallel tosaid beam axis.
 8. The method as claimed in claim 1, includingoscillatingly pivoting said workpiece surfaces around a said beam axis.9. The method as claimed in claim 1, wherein said fresh reactive gas isinlet substantially parallel to said beam axis.
 10. The method asclaimed in claim 1, wherein the distribution of the plasma density isset by means of a substantially axially parallel magnetic field.
 11. Themethod as claimed in claim 10, wherein the magnetic field has a strengthof between zero and 250 Gauss.
 12. The method as claimed in claim 11,wherein the magnetic field has a strength of 100 Gauss at most.
 13. Themethod as claimed in claim 12, wherein the magnetic field has a strengthof between zero and 60 Gauss.
 14. The method as claimed in claim 1,wherein said plasma beam is generated as a high-current arc by means ofa hot cathode discharge with cathode chamber with exit nozzle or with acold cathode.
 15. The method as claimed in claim 1, wherein saidworkpiece surfaces are at floating potential.
 16. The method as claimedin claim 1, wherein said workpiece surfaces are at a settable electricpotential that is at one of a DC, an AC, or an AC plus DC potential. 17.The method as claimed in claim 1, wherein said plasma beam has a lengthof more than 50 cm.
 18. The method as claimed in claim 1, includingdeposition at least one silicon compound as said coating material. 19.The method as claimed in claim 18, wherein said at least one siliconcompound is microcrystalline silicon μc-Si:H.
 20. The method as claimedin claim 19, wherein said reactive gas is silane.
 21. The method asclaimed in claim 20, wherein said the workpiece temperature is set bythe hydrogen content in container.
 22. The method as claimed in claim 1,wherein said coating-material forms hard substance or friction reducinglayers on said workpiece surfaces.
 23. The method as claimed in claim 1,wherein said workpieces are glass or silicon wafers.
 24. The method asclaimed in claim 1, wherein an arc current of 80 to 170A is set forgenerating said beam of a plasma and wherein said workpiece surfaces aredisposed at a distance r from said plasma beam axis so that 6 cm≦r≦20cm.
 25. The method as claimed in claim 24, wherein said distance r isset so that 9 cm≦r≦13 cm.
 26. The method as claimed in claim 1, wherein,in said container, a total pressure P_(tot) of 1 mbar≦P_(tot)≦3 mbar, isset.
 27. The method as claimed in claim 1, wherein workpiece surfacesare disposed in a plasma density region of said plasma beam of maximally20% of the maximum plasma density in the beam axis.
 28. The method asclaimed in claim 1, wherein said plasma beam is generated by one of ahot cathode or a cold cathode.
 29. The method as claimed in claim 1,wherein an arc current of 80 to 170A is set for generating said beam ofa plasma.